Set a line thirty minutes before bed. After it, the day is closed.
Behind that line, no new work. No starting tasks you cannot finish. No opening something that will pull you in.
What you do with the half hour is up to you. Read a few pages. Tidy one surface. Sit quietly. Keep it dull on purpose.
The boundary matters more than the activity. A clear edge between doing and resting tells the body that the day is winding down.
Use the same alarm you use for the morning, or set a quiet one for the evening. Let it mark the line so you do not have to watch the clock.
Thirty minutes is enough to notice and short enough to keep. Hold the line most nights and the rest tends to follow.